Electrical stimulation of auditory nerve fibers in persons with total sensory deafness has been found to produce auditory sensations which can be perceived with sufficient familiarity, that with minimal training such sensations can be used for speech reception without additional cues. Electrical stimulation of the auditory nerve fibers in deaf patients is accomplished in a number of methods. Many methods of stimulating the auditory nerve fibers utilize an array of electrodes mounted in a flexible worm-like carrier which is inserted into the cochlea of a patient's ear.
Because the human cochlea has a snail-like configuration, two basic mechanical designs have existed for intracochlear electrodes. The first design has a shape which matches the coiled structure of the human cochlea. A prosthesis of this design is temporarily straightened before insertion and regains its coiled shape upon insertion in the cochlea. The second design is a straight prosthesis which is very flexible, but which has sufficient stiffness to be guided into the cochlea in the desired coiled shape. The straight flexible configuration is often preferred. See Hochmair-Desoyer et al., Design and Fabrication of Multi-Wire Scala Tympani Electrodes, Vol. 405, Annals of the New York Academy of Sciences, pp. 173-182.
The straight flexible auditory prosthesis will typically have an array of electrodes positioned in one of a number of various configurations along the length of the prosthesis. These various configurations include monopolar and bipolar pairs of electrodes, which pairs may be positioned either radially or longitudinally along the flexible prosthesis. See Loeb et al., Design and Fabrication of an Experimental Cochlear Prosthesis, Medical & Biological Engineering and Computing, May 1983, Vol. 21, pp. 241-254; Clark et al., A Cochlear Implant Round Window Electrode Array, The Journal of Laryngology and Otology, Feb. 1979, Vol. 93, pp. 107-109.
An auditory prosthesis having a radial bipolar configuration appears to be the most favorable electrode geometry for the multiple electrode array. See, van den Honert et al., Single Fiber Mapping of Spacial Excitation Patterns in the Electrically Stimulated Auditory Nerve, Hearing Research, 29 (1987) pp. 195-206.
In addition to having a radial bipolar configuration, it is desirable that the array of electrodes have a large surface area with individual electrodes having a surface area of greater than 0.25 mm.sup.2. Further, the electrodes should have a decreased mass and volume so as to provide the desired mechanical characteristics to the prosthesis. The flexible prosthesis should also be capable of being easily fabricated.
Previous constructions have included so-called flame balls which are made by melting the ends of platinum or platinum iridium wires. Known practical flame ball constructions have longitudinal configurations as the volume of the flame balls does not permit radially placed electrodes. Another known configuration is that of electrode rings, which again are longitudinally spaced apart. Other known configurations include a mushroom shaped electrode having an outer domed surface approximating the surface of a flame ball protruding above the surface of the electrode body and with an inner stem anchoring the outer surface to the array. It is very difficult to place these electrodes radially, and, further, the large internal volume of the electrodes necessary to anchor them is undesirable.